A lithographic step using a resist is utilized recently in various fields.
In this lithographic step, a resist applied on a substrate or the like is partly removed by development and the exposed areas or remaining areas are suitably processed. This step is intended to thus impart a desired function to the corresponding areas of the substrate. After completion of the function impartation, the resist is removed.
In the production of a semiconductor device, for example, a resist material is applied on a semiconductor substrate such as a silicon wafer, and an image comprising a resist pattern is formed by ordinary photoprocessing. This resist pattern is used as a mask to subject the exposed areas to various treatments such as implantation of ions, e.g., P.sup.+, B.sup.+, or As.sup.+, and etching. In the implantation treatment, such ions are implanted also in an upper surface layer of the resist film image. Thereafter, the disused resist material is removed to form a predetermined circuit. Subsequently, a cycle consisting of the re-application of a resist material, image formation, etching, and the removal of the resist material is repeated in order to form a next circuit. Also in the case of forming a circuit on various substrates, the resist material disused after the formation of a resist pattern is removed.
The removal of a disused resist material is generally accomplished with an asher (ashing means), a solvent (remover), a chemical, etc. However, use of an asher for resist material removal is disadvantageous in that the operation needs much time, and that impurity ions contained in the resist material and the implanted ions remain on the semiconductor substrate and come into the semiconductor substrate as a result of a heat treatment conducted later. This inclusion of ions arouses a fear that a semiconductor integrated circuit may not be formed as designed, so that there are cases where a semiconductor device having impaired properties results. When an oxygen plasma is used as an ashing means, there are cases where the semiconductor substrate may be damaged, resulting in a semiconductor device having impaired reliability.
Furthermore, the wet cleaning method using a solvent or chemical (wet process) is disadvantageous in that much labor is required, for example, for preventing reverse fouling by the liquid used and for the disposal of the liquid after use. In particular, in APM cleaning with an NH.sub.4 OH/H.sub.2 O.sub.2 /H.sub.2 O liquid mixture, there is another problem that the silicon substrate is etched, and an improvement in this respect is desired. There has been still another problem that the working atmosphere is impaired.
Under these circumstances, the present applicants proposed, in official gazettes including Unexamined Published Japanese Patent Applications Nos. 4-345015 and 5-275324, a method for resist material removal which comprises applying an adhesive sheet in the form of sheet, tape, or the like to the upper surface of a resist pattern and then peeling the adhesive sheet as a united sheet including the resist material. This method is free from the problems that impurity ions contained in the resist material are implanted in the wafer and that the working atmosphere is impaired. This method is expected to be put to practical use as a simple and reliable method for removal.
However, an investigation made by the present inventors revealed that the above-described method using an adhesive sheet has the following problem. In the case where arsenic or other ions were implanted in a large dose (e.g., 1.times.10.sup.15 ions/cm.sup.2 or more) or in a large energy amount (e.g., 1 MeV or more) and this ion implantation has denatured a surface layer of the resist pattern, removability of the resist material with the adhesive sheet is deteriorated and it is difficult to completely peel off and remove the whole resist from the wafer surface.
In particular, in the case of ion implantation in a large energy amount, there has been a problem that it is extremely difficult to completely peel off and remove the whole resist from the wafer surface because the resist material to be removed has a relatively large thickness (about 2 to 5 .mu.m).
Accordingly, an object of the present invention is to provide a method capable of easily removing, without fail, a disused resist material from an article irrespective of the denaturation of a resist material surface layer caused by ion implantation, the thickness of the resist, etc., and to provide an adhesive or adhesive sheet for use in the method.
This sheet method has problems that impurities contained in the adhesive sheet diffuse into the resist film and then transfer to the substrate surface, and that impurities contained in the adhesive are directly transferred to the substrate surface in the areas where the adhesive sheet comes into direct contact with the substrate surface (the areas where the resist material is not present). That is, since the transferred substances contain metals such as alkali metals, alkaline earth metals, transition metals, and antimony, these metals pose problems such as junction leakage and a reduced charge retention time in DRAM, for example, in a semiconductor device if they remain on the substrate surface after completion of the step. Although light metals can be relatively effectively removed by conducting RCA cleaning or the like after use of the sheet method, the removal of transition and other metals remaining in a slight amount is extremely difficult. Hence, the above problems remain unsolved.
In view of these circumstances, another object of the present invention is to peel off a disused resist from an article, e.g., a silicon wafer, with an adhesive sheet while preventing impurities contained in the adhesive sheet from transferring to the surface of the article, e.g., wafer, and thus arousing electrical troubles resulting in problems such as decreases in the yield and reliability of the article.